Optical reproducing method for optical medium with aligned prepit portion

ABSTRACT

An optical reproducing method using an optical recording medium having an aligned prepit portion straddled on a plurality of tracks in a radial direction which includes a first and a second prepit portion divided in a track direction. The respective first and second prepit portion are arranged on a boundary of respective tracks and include a synchronous information prepit arranged at every two-track pitch in the radial direction and positioned at opposite boundaries of the same track. A trailing end of the first prepit portion is aligned with the radial direction in arrangement. the method includes irradiating an optical spot on the optical recording medium, detecting a reflected beam from the optical recording medium, and reproducing information on the optical recording medium by using a signal obtained in accordance with the reflected beam.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. application Ser. No.10/437,912, filed May 15, 2003, now U.S. Pat. No. 6,854,081, which is acontinuation of U.S. application Ser. No. 10/067,817, filed Feb. 8,2002, now U.S. Pat. No. 6,580,685, which is a continuation of U.S.application Ser. No. 09/809,048, filed Mar. 16, 2001, now U.S. Pat. No.6,392,985, which is with U.S. application Ser. No. 09/808,993, filedMar. 16, 2001, now U.S. Pat. No. 6,370,106, which are continuations ofU.S. application Ser. No. 09/514,284, filed Feb. 28, 2000, now U.S. Pat.No. 6,262,968, which is a continuation of U.S. application Ser. No.09/181,677, filed Oct. 29, 1998, now U.S. Pat. No. 6,064,644, which is acontinuation of U.S. application Ser. No. 08/958,867, filed Oct. 27,1997, now U.S. Pat. No. 5,898,663, which is a continuation applicationof U.S. application Ser. No. 08/733,924, filed Oct. 18, 1996, now U.S.Pat. No. 5,982,738, which is a continuation-in-part application of U.S.application Ser. No. 08/600,730, filed Feb. 13, 1996, now U.S. Pat. No.5,805,565, the subject matter of which are incorporated by referenceherein.

BACKGROUND OF THE INVENTION

The present invention relates to an optical recording medium and moreparticularly to a high-density optical recording medium having a trackwidth smaller than an optical spot diameter.

An example of a medium for performing high-density (narrow track)recording is disclosed in, for example, JP-A-6-176404. According to thisexample, in an optical recording medium having grooves and lands whichare formed on a substrate and information recording areas which areformed in association with both the groove and the land, prepits aredisposed on a virtual extension line of the boundary between a grooveand a land. In particular, the prepits are located on only one side ofany specific position of the center line of each groove.

With this construction, recording information is formed on both thegroove and the land, the prepits have charge of address datarepresentative of the recording areas and one prepit is used in commonto a pair of adjacent groove and land to provide address data therefor.When the technique as above is applied to, for example, a phase changerecording medium or a magneto-optical recording medium, interference ofinformation (crosstalk) between adjacent lands or grooves due to theoptical interference effect within an optical spot can be prevented,thereby permitting narrowing of track.

On the other hand, in the prepit area free from the optical interferenceeffect, the address data can be common to the paired groove and land andthe effective track pitch can be increased to reduce crosstalk.

In the example of JP-A-6-176404, however, the disposition of the prepitarea is offset on one side of the center line of the groove or land, sothat when an optical spot is caused to track a groove or a land, atracking error (tracking offset) increases, making it difficult toperform high-density recording in which the track pitch is narrowed.

SUMMARY OF THE INVENTION

The present invention achieves elimination of the above problems and itis a first object of the present invention to provide an opticalrecording medium which can suppress the tracking offset to a value orlevel which is sufficiently low for the practical use and permitefficient disposition of address data even when recording is effected onboth the groove and the land.

A second object of the present invention is to provide a high-densityoptical recording medium which can ensure simple mastering and easyreplica preparation and can permit decoding even when a readout errortakes place.

To accomplish the above first object, the following expedients areemployed.

(1) Grooves and lands are formed on a substrate of a recording medium,information recording areas are formed in association with both thegroove and the land, and prepits are disposed on a virtual extensionline of the boundary between a groove and a land.

The disposition of prepits satisfies the following requirements (a) to(c) at the same time.

(a) Prepits are located on both sides of a virtual extension of thecenter line of a groove;

(b) Prepits are located on both sides of a virtual extension of thecenter line of a land;

(c) Prepits are not located on the both sides of any specific positionof the center line of a groove; and

(d) Prepits are not located on the both sides of any specific positionof a land.

With this construction, the arrangement of prepits is not offset oneither one side of a virtual extension of the center line of the grooveor the land to ensure that tracking offset hardly occurs and the prepitsdo not exist on both sides of any specific position of the center lineof the groove or the land to prevent interference of prepit informationbetween adjacent tracks from taking place within a reproduced spot so asto permit high-density narrow track recording.

(2) When prepits are disposed in the circumferential direction such thatthose on one side of a groove are not discriminative from those on theother side or those on one side of a land are not discriminative fromthose on the other side, at least consecutive two dispositions ofprepits associated with the groove or the land are made to be differentfrom each other to provide the same disposition of prepits periodicallyevery two dispositions.

As the other option,

(3) A groove associated with at least one pair of pits disposed on bothsides of the center line of the groove in a prepit area and an adjacentgroove not associated with pits disposed on both sides of the centerline of this groove within the prepit area are disposed alternately inthe radial direction.

Through this, by merely reproducing the pits, prepits associated withthe groove can be discriminated from those associated with the land toimprove reliability of information recording reproduction.

(4) Either one of synchronous information and address data isrepresented by prepits disposed on either one of the both sides of agroove.

As the other option,

(5) Only one of synchronous information and address data is representedby prepits arranged on one side of a groove and both the synchronousinformation and the address data are represented by prepits arranged onthe other of the both side of the groove.

Through this, address data can be reproduced under accuratesynchronization. In addition, since the phase margin between prepits onthe both sides can be extended, fabrication of a recording medium can befacilitated.

(6) The groove and the prepit have the same depth which is 70 nm orless. More preferably, the depth is 40 nm or more and 60 nm or less.

With this construction, an advantage of suitable crosstalk cancellationcan be obtained between the groove and the land and besides an excellenttracking servo signal can be obtained. Formation and fabrication of therecording medium can be facilitated. With the groove depth exceeding 70nm, the formation of the groove is difficult to achieve. When the groovedepth is about 50 nm, the tracking servo is maximized and with thegroove depth being about 50±10 nm, substantially the same effect can beattained.

(7) The groove and the land have substantially the same width which isbetween 0.3 μm and 0.75 μm.

With this construction, excellent tracking is compatible withhigh-density recording. If the groove and land have a width of notgreater than 0.3 μm, then two of the groove and land will be confinedwithin one optical spot and an excellent tracking signal cannot beobtained. On the other hand, if the groove and land have a widthexceeding 0.75 μm, then effective high-density recording cannot bepermitted.

(8) Of prepits, the smallest one has a diameter which is smaller than awidth of each of the groove and the land. More preferably, the diameteris in the range from 0.25 μm to 0.55 μm.

Through this, an excellent prepit signal can be obtained withoutcrosstalk. With the diameter being not greater than 0.25 μm, the prepitsignal decreases in the extreme and with the diameter exceeding 0.55 μm,crosstalk is generated.

In the present invention, prepits are arranged on the both sides of avirtual extension line of the center line of a groove or a land in theoptical spot scanning direction. Consequently, offset is decreased tomake the tracking offset hardly occur and the prepits do not exist onthe both sides of any specific position of the center line of the grooveor the land to ensure that interference of prepit information betweenadjacent tracks within a reproduced spot can be prevented andhigh-density narrow track recording can be permitted.

Further, even in the presence of tracking offset, the amount of trackingoffset can be detected accurately by comparing amplitudes of signalsrepresentative of prepits on the both sides. Accordingly, byfeedback-controlling the information indicative of a comparison resultto a scanning unit, the tracking offset can be suppressed.

At a portion between a groove and a prepit area, between a land and aprepit area or between prepit areas, a gap takes place when a prepittrain on a virtual extension line of the boundary between a groove and aland shifts to a prepit train on an adjacent virtual extension line. Theaforementioned JP-A-6-176404, however, does not take the gap intoconsideration. Accordingly, in the absence of the gap or with the gapbeing very short, mastering of the substrate cannot be proceeded with byone-beam cutting and requires two-beam cutting. Further, during replicapreparation, injection must be applied to a steep pattern, leading to adecrease in yield. In addition, during reproduction of signals,tolerance to distortion of the reproduced spot and the tracking offsetis decreased and a readout error is liable to occur.

To accomplish the second object, the following expedients are employed.

(1) Grooves and lands are formed on a substrate and prepits are arrangedon a virtual extension of the boundary between a groove and a land. Inparticular, the prepits are disposed on both sides of an extension ofthe center line of a groove or a land and therefore, the optical axis ofa laser beam must be moved during cutting. An acoustic-optical deflector(AOD) is used to change the optical axis. But it takes a time for theAOD to cause the optical axis to reach a desired optical axis positionafter transmitting a signal for optical axis change and when a modulatedlaser beam is irradiated along the intact optical axis, pits are formedobliquely on the substrate. Accordingly, no pattern is formatted betweenthe groove or the land and the succeeding prepits to provide a gap andan acoustic-optical modulator (AOM) is cut off corresponding to the gapto prevent laser irradiation and pit drawing. Thus, the substrate can befabricated with a simple cutting machine. In addition, since a number ofunevennesses are not formed on a narrow area on the substrate, the yieldduring preparation of replica can be increased.

(2) In the disposition in which prepits are arranged on a virtualextension of the boundary between a groove and a land, when thedisposition of a prepit train on one side of a virtual extension of theboundary between the groove and the land is exchanged with thedisposition of a prepit train on the other side or vice versa, thetrailing edges of prepit trains on the respective one sides are alignedwith each other in the radial direction of the substrate. The succeedingpit trains are spaced from those trailing edge positions in thecircumferential direction or the recording/reproducing direction and thetrailing edges of the succeeding pit trains are aligned with each othersimilarly. When the formed gap meets the recording rule, the substrateas a whole can be formatted conveniently and portions devoid of pits canbe collected at a specified area on the substrate, thereby solvingproblems involved in cutting and replica preparation for reasonsdescribed previously.

(3) Radially adjacent pit trains each having only original informationpits cannot be aligned with each other at the trailing edge in theradial direction. Accordingly, new pits are added to ensure thealignment of the trailing edges in the radial direction while observingthe rule during recording.

(4) In the shift of the disposition of a pit train from one side to theother as described in the above (2), leading edges of pits in thedisposition on the other side can be aligned in the radial direction tosolve the problems involved in cutting and replica preparation for thesame reasons set forth in the (2). In particular, from the standpoint ofsignal reproduction, a synchronous signal is allotted to pit informationimmediately after the shift of the pit train so that decision of achannel bit at the specified position may be thought much of, therebyensuring that the tolerance to the leading edge position can beincreased and a possibility that erroneous reading of important data of,for example, address at the position immediately before the shift of apit train can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged fragmentary plan view of a first embodiment of anoptical recording medium according to the present invention.

FIG. 2 is a waveform diagram of reproduced signals from the medium ofFIG. 1.

FIG. 3 is a block diagram of an apparatus for recording and reproductionof the optical recording medium used in the present invention.

FIG. 4 is an enlarged fragmentary plan view of a second embodiment ofthe optical recording medium according to the present invention.

FIG. 5 is an enlarged fragmentary plan view of a third embodiment of theoptical recording medium according to the present invention.

FIG. 6 is a similar view of a fourth embodiment of the optical recordingmedium according to the present invention.

FIG. 7 is a waveform diagram of reproduced signals from the opticalrecording medium of FIG. 6.

FIG. 8 is an enlarged fragmentary plan view of a fifth embodiment of theoptical recording medium according to the present invention.

FIG. 9 is a diagram showing an information structure in the fifthembodiment of the optical recording medium according to the presentinvention.

FIG. 10 is an enlarged fragmentary perspective view showing the relationbetween the prepit area and the groove in the fifth embodiment.

FIG. 11 is an enlarged fragmentary plan view showing details ofpositional displacement in the embodiments of the optical recordingmedium according to the present invention.

FIG. 12 is an enlarged fragmentary plan view of a sixth embodiment ofthe optical recording medium according to the present invention.

FIG. 13 is a diagram showing an example of a modulated code in the sixthembodiment of the optical recording medium according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1 (Optical Recording Medium)

Referring now to FIG. 1, there is illustrated, in enlarged fragmentaryplan view form, an optical recording medium of the present invention.Grooves 84 each having a width of 0.6 μm and a depth of 50 nm and lands85 each having a width of 0.6 μm are arranged alternately in the radialdirection of the medium and recorded marks 81 are formed on the twokinds of areas. In other words, each of the groove 84 and land 85 servesas a recording area. In a prepit area 83, any groove is not formed butpits 82 are disposed on an extension of the boundary between a land anda groove. Each of the pits has a width of 0.35 μm and a depth of 50 nm.The prepit area is divided into a first prepit area 831 and a secondprepit area 832. In the first prepit region 831, pits 82 are disposed onthe upper side, as viewed in the drawing, of the center line of the land85 and in the second prepit area 832, pits 82 are disposed on the lowerside, as viewed in the drawing, of the center line of the land 85.Accordingly, when an optical spot 21 scans, for example, the land 85,pits on only either one of the sides are always reproduced and there isno fear that crosstalk will occur between adjacent tracks. Therefore,address data recorded in the form of the prepits can duly be reproducedwithout crosstalk.

Since the pits 82 do not adjoin to each other in the radial direction,they can be formed with ease. Also, pits 82 are uniformly disposed onboth sides of a track (a land or a groove) and hence the influence on atracking servo signal, which is caused by the pits 82, can be canceled.Accordingly, the tracking offset can be suppressed to a sufficientlysmall level.

Further, when reproducing, for example, a land 85, reproduction ofaddress data at the second prepit area 832 is carried out continuouslywith reproduction of address data at the first prepit area 831.Accordingly, when the two areas are united into one area in whichinformation is arranged to provide address data for one track, anaddress (track number) of a land and that of a groove can be setindependently of each other. In other words, by sequentially reproducingthe address data pieces in the first and second prepit areas 831 and832, discrimination between the land and the groove can be ensured.

More particularly, for reproduction of the groove, address datarepresented by prepits arranged in the first prepit area is made to beidentical to that represented by prepits arranged in the second prepitarea but for reproduction of the land, address data represented byprepits in the first prepit area is made to be different from thatrepresented by prepits in the second area. When addresses represented byprepits in the first and second prepit areas are different from eachother, a correlation may be set up between the two addresses and theefficiency of error correction code can be increased by utilizing thecorrelation.

Preferably, synchronous information (VFO) 86 and address data 87 mayboth be arranged in each of the first and second prepit areas.

While in this example the prepit area is divided into two of the firstand second prepit areas, the number of division which is plural maysuffice. For example, when the number of division is four, pits in firstand third prepit areas may be arranged on one side of a groove and pitsin the second and fourth prepit areas may be arranged on the other sideof the groove. By increasing the number of division of the prepit area,reliability against, for example, defects can be improved.

Here, a phase change recording material (GeSbTe) is used for therecording film. Accordingly, the recorded mark is formed in the form ofan amorphous domain.

Referring now to FIG. 3, there is illustrated an example of aconfiguration in which the optical recording medium of the presentinvention is applied to an optical recording/reproducing apparatus. Inthe apparatus, a semiconductor laser 311 having a wavelength of 680 nmand a collimating lens 312 are used as a light source 31. A beam profileformer such as a prism may also be provided as necessary. Power of thesemiconductor laser is controlled by a light power controller 71 havingthe auto-light-power-control function. A light beam 22 emitted from thelight source 31 is focused on a magneto-optical recording medium 8 bymeans of a focusing optics 32. The focusing optics 32 has at least onelens 321 and in this example, it also has a beam splitter 324. Anobjective lens 321 for focusing the light beam on the optical recordingmedium 8 is designed to have a numerical aperture of 0.6. Therefore, anoptical spot 21 on the optical recording medium 8 has a diameter of 1.0μm. The optical spot can be moved to a desired position on the opticalrecording medium 8 by means of a scanning unit 6. The scanning unit 6includes at least a motor 62 for rotating the disc-like magneto-opticalrecording medium 8 and an auto-position controller 61 having thefunction of auto-focus control and auto-tracking. The auto-positioncontroller 61 utilizes a reflected beam 23 from the magneto-opticalrecording medium 8 to cause a photodetection unit 33 to detect anoptical spot position which is used for feedback control.

The optical spot position can be detected by detecting power of adiffracted light ray from a groove. The photodetection unit 33 isconstructed of a lens, a beam splitter and a plurality ofphotodetectors, and output signals of the plurality of photodetectorsare calculated to produce a servo signal and a reproduced signal.

With the optical recording medium as shown in FIG. 1 used, signals asdesignated at 14 in FIG. 2 are produced as prepit signals. The signal isinputted to an address detection unit to decode address data and at thesame time, timings of signals of the first and second prepit areas aredetected and on the basis of the timing information, the amplitude(averaged peak-to-peak amplitude) of the first prepit area and that ofthe second prepit area are stored. The thus stored amplitudes arecompared with each other by means of an amplitude comparator to producetracking offset information which in turn is fed back to the positionmoving unit (scanning unit). Referring to FIG. 2, when the optical spotscans a groove, a magneto-optical reproduction signal 11 and acorresponding prepit signal 14 (an upper one in the drawing) areproduced and when the optical spot scans a land, a magneto-opticalreproduction signal 12 and a corresponding prepit signal 14 (a lower onein the drawing) are produced. Since in this example the optical spot isslightly offset as shown in FIG. 1, an amplitude difference 13 takesplace between a prepit signal from the first prepit area 831 and aprepit signal from the second prepit area 832. This amplitude differencecorresponds to a tracking offset amount.

By using the apparatus of FIG. 3, the tracking offset could be reducedto ±0.03 μm or less even when various kinds of external disturbance suchas aberration of the optical spot are taken into consideration. Underthe nominal state devoid of optical aberration, the tracking offset was±0.015 μm or less.

As described above, in the present invention, prepits are disposed onboth sides of a virtual extension of the center line of a groove or aland as shown in FIG. 1. Consequently, offset is reduced to maketracking offset hardly occur. Since prepits do not exist on both sidesof any specific position of the center line of a groove or a land,interference of prepit information between adjacent tracks does not takeplace within a reproduced spot and hence high-density narrow trackrecording can be ensured.

Further, if a tracking offset occurs as shown in FIG. 2, the trackingoffset amount can be detected accurately by comparing amplitudes ofsignals of prepits located on both sides. Accordingly, byfeedback-controlling the information indicative of a comparison resultto the scanning unit, the tracking offset can be suppressed.

Furthermore, discrimination between the groove and the land can beeffected with ease.

By using the optical recording medium of the present invention, thetracking offset can be suppressed to a practically sufficiently smalllevel (0.03 μm or less) and besides, address data can easily be obtainedeven during high-density narrow track recording. Through the use of theoptical recording/reproducing apparatus of the present invention, thetracking offset can readily be reduced by feedback control.

Embodiment 2

Referring now to FIG. 4, there is illustrated a second embodiment of thepresent invention. A medium of the present embodiment differs from thatof embodiment 1 in that only synchronous information pits 861 to 864 aredisposed on the upper side (as viewed in the drawing) of the center lineof a groove 841, 842, 843, 844 or 845 and synchronous information pits861 to 864 and address data pits 871 to 874 are both disposed on thelower side (as viewed in the drawing) of the center line of each of thegrooves 84. Preferably, the address data pits 871 to 874 are arrangedcontinuously to the synchronous information pits 861 to 864. For a land85, the upper and lower side relation is inverted.

Being different from the embodiment 1, the present embodiment hasaddress data arranged on only the upper or lower side of the center lineof the groove or the land and therefore the same address data isallotted to the land and groove. In the present embodiment, fourdivisional prepits areas 831 to 834 are provided with the aim ofimproving the reliability of the prepit area but the prepit area is notalways divided. In the present embodiment, the synchronous pit 861 inthe first prepit area 831 are designed to have a longer length than thesynchronous pits in the second to fourth prepit areas by taking intoaccount the influence of aliasing of a signal which has passed through alow pass filter. Preferably, pits disposed on the upper and lower sidesare spaced apart from each other by 0.5 μm or more from the view-pointof fabrication of the medium. More preferably, they are spaced apart bya distance of about 1 μm which is the diameter of the reproduced opticaldisc spot.

Embodiment 3

Referring to FIG. 5, there is illustrated a third embodiment in whichidentification marks 88 are used to discriminate the land from thegroove. In the present embodiment, identification marks 88 fordiscrimination between the land and the groove are providedindependently of the prepit area in the embodiments 1 and 2.

In the present embodiment, a pair of pits (identification marks) 88 arearranged on the upper and lower (as viewed in the drawing) sides of thecenter line of a groove 841, 843 or 845 but they are not provided for agroove 842 or 844. On the assumption that an optical spot relativelymoves from left to right as viewed in the drawing when the mediumprovided with the above identification marks is reproduced to provide acase of “presence” where the identification marks are seen by theoptical spot and a case of “absence” where the identification marks arenot seen, “presence, presence” is held for the groove 841, “absence,presence” is held for a land 851, “absence, absence” is held for thegroove 842 and “presence, absence” is held for a land 852. Further,“presence, presence” is held for the groove 843, “absence, presence” isheld for a land 853, “absence, absence” is held for the groove 844 and“presence, absence” is held for a land 854. Namely, either one of“presence, presence” and “absence, absence” is held for the groove andeither one of “absence, presence” and “presence, absence” is held forthe land. Accordingly, this can be utilized to effect discriminationbetween the land and groove on the basis of a reproduced signal. Tosecure reliability, a plurality of pairs of identification marks maypreferably be provided and more preferably, the paired pits are spacedapart from each other by several μm or more in the circumferentialdirection or information track direction of the medium which isperpendicular to the radial direction. For example, the prepit area inthe foregoing embodiments and the identification mark area maypreferably be arranged alternately in the circumferential direction.

Embodiment 4

Referring to FIG. 6, there is illustrated, in enlarged fragmentary planview form, an optical recording medium according to a fourth embodimentof the present invention. Grooves 84 each having a width of 0.5 μm and adepth of 40 nm and lands 85 each having a width of 0.5 μm are arrangedalternately and recorded marks 81 are formed on the two kinds of areas.In other words, each of the land 85 and groove 84 serves as a recordingarea. In a prepit region 83, any groove is not formed but substantiallycircular pits 82 (each having a diameter of 0.3 μm and a depth of 40 nm)are disposed on an extension of the boundary between a land and agroove. The prepit area is divided into a VFO (variable frequencyoscillator) area 833 and an address area 834. Especially, in the VFOarea, pits 82 are disposed alternately on the upper and lower sides ofthe center line of a land 85. In the address area, pits 82 are disposedalternately at the same period as that in the VFO area. Accordingly,there are no pits which exist on both sides of a position of the centerline of the land or the groove.

In addition, in the address area, data for a particular track is soencoded as to differ by one pit from data for an adjacent track. Inother words, the data takes the form of a Gray code. With thisconstruction, when an optical spot 21 scans, for example, a land 85,only pits on either one side are always reproduced and there is no fearthat crosstalk will occur between the adjacent tracks. Therefore,address data distributed to the prepits can duly be reproduced withoutcrosstalk. Since pits 82 for adjacent tracks do not adjoin to eachother, they can therefore be formed with ease. Also, pits 82 areuniformly disposed on both sides of a track (a land or a groove) andhence the influence on a tracking servo signal which is caused by thepits 82 can be canceled. Accordingly, tracking offset can be suppressedto a minimum.

When the medium of the FIG. 6 embodiment is reproduced with theapparatus of FIG. 3, reproduced signals as shown in FIG. 7 are generatedfrom the prepit area 83, indicating that data pieces which differ trackby track can be obtained and therefore address data is recorded veryhighly efficiently. Thanks to the use of the Gray code, an address canbe reproduced even in the course of inter-track access, ensuringsuitability to high-speed access. Further, with the Gray code used, anerror hardly occurs even in the presence of crosstalk, thus ensuringsuitability to narrowing of tracks.

Embodiment 5

Referring now to FIG. 8, there is illustrated in enlarged fragmentaryplan view form an optical recording medium according to a fifthembodiment 5 of the present invention. Groove 84 each having a width of0.7 μm and a depth of 70 nm and lands 85 each having a width of 0.7 μmare arranged alternately in the radial direction and the two kinds ofareas serve as information tracks on which recorded marks can be formed.

In other words, each of the land 85 and groove 84 serves as a recordingarea. In a prepit region 83, any groove is not formed but pits 82 aredisposed on an extension of the boundary between the land and thegroove. The prepit area is divided into zones which are arranged in theradial direction over about 1800 information tracks, that is, 900grooves.

The zones are arranged concentrically of the whole of a disc in such amanner that 24 zones in total are in a disc having a radius of 30 to 60mm. More specifically, in each zone, the number of prepit areas to bedetected during one revolution, that is, the sector number is constantand the sector number is larger in an outer zone than in an inner zone.

An example of structure of each sector 41 is shown in FIG. 9. The sector41 has a prepit area 83 at the head of a data recording area.

As shown in FIG. 8, the prepit area is divided into a first prepit area831 and a second prepit area 832. In the first prepit area 831, pits 82are arranged on the upper side (as viewed in the drawing) of the centerline of a land 85 and in the second prepit area 832, pits 82 arearranged on the lower side of the center line of the land 85.Consequently, for example, when an optical spot 21 scans the land 85,only pits on either one side are always reproduced and there is no fearthat crosstalk will occur between adjacent tracks. Accordingly, addressdata allotted to the prepits can duly be reproduced without crosstalk.Address data represented by the prepits is recorded in the form of a 1–7modulation code (having a channel bit length of 0.2 μm). In other words,the linear recording density is 0.3 μm/bit.

The relation between the prepit area and the groove in the presentembodiment is illustrated in enlarged fragmentary section perspectiveview form in FIG. 10.

In the present embodiment, a gap area 87 is provided between the firstand second prepit areas 831 and 832 to space them apart by about 1.0 μm.Since in this embodiment data is recorded pursuant to the 1–7 recording,the gap distance corresponds to a length of about 5 channel bits. The 5channel bit length is exactly the middle length between the longest marklength (8 channel bit length) and the shortest mark length (2 channelbit length). Therefore, the gap area between the first and second prepitareas can be reproduced having a length which lies between the shortestmark length and the longest mark length even when the pits undergochanges in shape and position during formation of the pits and theoptical spot undergoes a change in shape and a change in scanningposition (servo offset) thus ensuring very high reliability. In thisexample, the marks are designed to undergo, at the worst, a total changein position which is suppressed to 0.6 μm (3 channel bit length) andtherefore, the effective length (during reproduction) is 2 channel bitsin the case of the shortest length and 8 channel bits in the case of thelongest length to match the rule of the 1–7 modulation code, thusraising no problem during reproduction. If the detection length islonger than 8 channel bit length, then it will adversely interfere witha special synchronous pattern such as a recorded address mark. If thedetection length is shorter than 2 channel bits, then a small markresults which is less than resolution of the reproduction optical spotand cannot be detected. Accordingly, it is preferable that the gaplength be suppressed to the middle between the longest mark length andthe shortest mark length as in the present embodiment.

Depending on the specification of a pit forming apparatus, the change inmark position can be suppressed to one channel bit length or less. Inthis case, the nominal gap length may be suppressed to 3 to 7 channelbit length but the pit forming apparatus for this purpose becomesexpensive. There is a high possibility that signals suffer an errorattributable to a tracking offset during reproduction and therefore themedium is desired in which preferably, the gap length is exactly themiddle between the longest mark length and the shortest mark lengthpermissible for the recording as described hereinbefore.

In the present embodiment, pits 82 are uniformly disposed on both sidesof the center line of a track (a land or a groove) and hence theinfluence on a tracking servo signal which is caused by the pits 82 canbe canceled. Accordingly, the tracking offset can be suppressed to asufficiently small level. In addition, for example, when a land 85 isreproduced, reproduction of address data at the second prepit area 832is carried out continuously with reproduction of address data at thefirst prepit area region 831. Accordingly, when the two areas are unitedinto one area in which information is arranged to provide address datafor one track, an address (track number) of a land and that of a groovecan be set independently of each other. In other words, by sequentiallyreproducing the address data pieces in the first and second prepit areas831 and 832, discrimination between the land and the groove can beensured.

More particularly, for reproduction of the groove, address datarepresented by prepits arranged in the first prepit area is made to beidentical to that represented by prepits arranged in the second prepitarea but for reproduction of the land, address data represented byprepits in the first prepit area is made to be different from thatrepresented by prepits in the second area. When addresses represented byprepits in the first and second prepit areas are different from eachother, a correlation may be set up between the two addresses and theefficiency of error correction code can be increased by utilizing thecorrelation.

Preferably, synchronous information (VFO) 86 and address data 87 mayboth be arranged in each of the first and second prepit regions.

While in this example the prepit area is divided into two of the firstand second prepit areas, the number of division which is plural maysuffice. For example, when the number of division is four as shown inFIG. 5, pits in the first and third prepit areas may be arranged on oneside of a groove and pits in the second and fourth prepit areas may bearranged on the other side of the groove. By increasing the number ofdivision of the prepit area, reliability against, for example, defectscan be improved.

Here, a phase change recording material (GeSbTe) is used for therecording film. Accordingly, the recorded mark is formed in the form ofan amorphous domain.

Referring now to FIG. 11, amounts of positional displacement 963 betweenprepit areas of adjacent tracks, 961 between prepits of adjacent tracksand 962 between grooves of adjacent tracks in the medium are illustratedin greater detail. In the actual medium, positional displacementsometimes occurs between pits of adjacent tracks owing to various causestaking place during pit formation. Because of the positionaldisplacement amounts 961, 962 and 963, the length of gap areas 86 and 87is increased or decreased.

In addition to the above positional displacement, various kinds ofvariations (aberration, servo error and the like) during reproductionalso cause apparent positional displacement of reproduced signals.Accordingly, the positional displacement possibly leads to a seriousproblem. But in the present invention, the nominal length of the gaparea is set to the middle length between the shortest mark length andthe longest mark length pursuant to the 1–7 modulation code and hence apositional displacement amount of ±0.6 μm is permissible.

The optical recording medium shown in FIG. 8 can be reproduced with theapparatus shown in FIG. 3 in a similar manner to that described inconnection with embodiment 1, bringing about advantages that trackingoffset can be reduced to ±0.03 μm or less even when various kinds ofexternal disturbance such as optical aberration are taken into accountand in particular, it can be reduced to ±0.015 μm or less under thenominal state devoid of optical aberration.

Embodiment 6

While the embodiment of FIG. 8 uses the 1–7 modulation coding as therecording modulation coding, the present embodiment uses eight tofourteen modulation (EFM) recording. The channel bit length is about 0.2μm. In the present recording, the shortest mark length is 3 channel bitlength and the longest mark length is 11 channel bit length.Practically, a mark having a length of 12 channel bits or more isavailable but this type of mark is limited to a special application suchas a synchronous pattern. Accordingly, data must avoid inclusion of apattern which may possibly interfere with the special pattern. Theprepit area, groove and land are arranged similarly to the embodiment 5of FIG. 8 excepting points to be described later. Namely, they arearranged as shown in FIG. 10 and especially, each groove and each have awidth of about 0.75 μm and each groove and each prepit have a depth ofabout 0.075 μm.

In the present embodiment, the prepit area and the groove are disposedas shown in FIG. 12. Four prepit areas 831, 832, 833 and 834 areallotted to the head of one sector. In each prepit area, a VFO area forsynchronization to reproduced signals and an address area recordingaddress data of the track and sector are arranged sequentially. Startpositions of pits as well as end positions are so arranged as to besubstantially aligned in the radial direction and a gap area 86 isprovided between the end of the groove and the start of the pit area.Likewise, a gap area 87, 88 or 89 is provided between adjacent prepitareas.

As described previously, details of positional displacement between thestart position of a pit and the start position of a succeeding pit isdepicted in FIG. 11. With the displacement as shown in FIG. 11, theeffective length of the gap area 86 is decreased,or increased as in theembodiment 5 of FIG. 8. In order to align ends of final pits of theprepit areas 831, 832 and 833 in the radial direction, an additional pitpattern 110 as shown in FIG. 13 is used. The additional pattern selectedfrom four types (a), (b), (c) and (d) in accordance with the precedingdata is used. Through this, trailing edge positions 99 of the final pitscan always be aligned to the same position regardless of the precedingdata and the gap length and pit length can be limited to the lengthsallowed for the modulation code. In this manner, the gap area betweenthe succeeding prepit area 120 and the trailing edge position 99 of thefinal pit can be set to the middle length between the longest marklength and the shortest mark length pursuant to the modulation coding,so that the margin can be greatly increased during prepit formation andreproduction as in the embodiment 5.

In the foregoing embodiments, the medium of the phase change recordingmaterial is described but it may be of another material to attain theadvantages of the present invention. For example, a magneto-opticalrecording film may be used as the recording film. In addition, themodulation code has been described as being of 2–7 and 8/9 coding but itmay be of another type in which the previously described EFM isextended.

According to the present invention, in the optical recording mediumhaving the lands and grooves, the substrate can be fabricated with asimple mastering apparatus and replica can also be prepared with ease,with the result that the medium fabrication margin and readout marginwhich are practically sufficiently large can be ensured. Accordingly, acheap and high-density optical recording medium can be provided.

1. An optical reproducing method using an optical recording mediumhaving an aligned prepit portion straddled on a plurality of tracks in aradial direction; the prepit portion including a first and a secondprepit portion divided in a track direction; the respective first andsecond prepit portions being arranged on a boundary of respectivetracks; the first prepit portion including a synchronous informationprepit arranged at every two-track pitch in the radial direction, thesecond prepit portion including a synchronous information prepitarranged at every two-track pitch in the radius direction, and thesynchronous information prepit of the first prepit portion and thesynchronous information prepit of the second prepit portion beingarranged at opposite boundaries of the same track; a trailing end of thefirst prepit portion being aligned with the radial direction inarrangement; the optical reproducing method comprising the steps of:irradiating an optical spot on the optical recording medium; detecting areflected beam from the optical recording medium; and reproducinginformation on the optical recording medium by using a signal obtainedin accordance with the reflected beam.